Yep, they do indeed seal at times. I've never heard the lids 'pop' as I have all of my jars in the basement in a very large cooler (damn cold in the winter). But, it really doesn't make any difference to me as I eventually pop them open when I wish to delve into a particular jar and smoke a bowl. However, if by chance the lid pops down that does not mean you cannot use it again as the suction created is not that strong and they will, eventually, 'collapse' again. Even if you feel that the lid has lost it's integrity to 'collapse', as it were, does not necessarily mean that it is useless to keep the jar sealed. But, when in doubt purchase new lids as they are very cheap whereas tobacco is not! :wink: